Process of chemically treating and bleaching cellulose pulp



Patented May 17, 1932 UNITED STATES PATENT OFFICE:

GEORGE A. RICHTER AND MILTON O. SCHUR, OF BERLIN, NEW HAMPSHIRE, ASSIGNORS TO BROWN COMPANY OF BERLIN, HAMPSHIRE, A CORPORATION OF MAINE PROCESS OF CHEMICALLY TREATING AND BLEACHING- CELLULOSE PULP No Drawing. Original application filed June 6, 1928, Serial No. 283,452. Divided and this application filed August 16, 1929. Serial No. 386,487.

10 lose content of the pulp through the formation of oxycelluloses. The free alkali is preferably added to the bleach liquor at the start of bleaching and somewhat in excess of the amount necessary to be gradually consumed by reaction with acidic constituents initially present in the unbleached pulp or formed during bleaching, so that a distinctly alkaline condition is maintained throughout the bleaching operation; The alkali apparently makes the reaction between the bleach and pulp more selective in so far as the removal of impurities from the pulp isconcerned. It also appears to have a solvent efi'ect upon such oxycelluloses as may be present in the pulp at the start of bleachin or as may be formed during bleaching. his patent further teaches the fact that higher temperature may be safely used during bleaching it free alkali be present in the bleach liquor, thereby eliminating the necessity for refrigerating the bleach liquor in hot weather, especially when stock is being bleached at a high density. Because of the protective influence which the alkali exercises over the cellulose, danger of local overbleaching is avoided, so that raw stock and strong bleach liquor may be uniformly admixed even in the summer when local overbleaching would ordinarily be o'ertain to occur, particularly during initial mixing of the comparatively high temperature stock, resulting in a weakened stock and in a comparatively high bleach usage. A further advantage resulting from the use of free alkali during bleaching is the reduction inthe number of resinous specks usually associated .with the fibers, and accordingly the production of a cleaner pulp.

We have now found that if an alkali such as caustic soda is added to the raw stock before the bleach liquor, and is allowed to rethe bleach.

main in contact therewith at about room temas calcium hypochlorite, is subsequently em- I h ployed, for when the alkali and calcium hypochlorite are added simultaneously to the raw pulp, there is a tendency for the bleach to react with the resins to form insoluble rcsinates and oxidation products which become fixed on the pulp, thereby interfering to an appreciable extent with the solutionor dispersion of the resins, and the attainment of high whiteness. Even when sodium hypochlorite bleach is subsequently used, the initial treatment with an alkali such as'caustic soda is advantageous, in that the resins are peptized and brought into a condition more susceptible to attack and removal by In our application Serial No. 283,452, filed June 6, 1928, various examples of initially treating the stock with alkali and then bleaching were given, one of these examples involving the addition of bleach to the alkaline liquor after the stock has been in contactwith such alkaline liquor for a while. This application is a divisional of our application Serial No. 283,452, and is intended to cover such a procedure, which is especially advantageous in a mill where a continuous process is being practised, that is, Where the pulp is caused to how as a continuous stream of pulp suspension through a suitable system or apparatus. In such case,

about caustic soda may. be added to the raw stock, e. g., sulphite pulp, entering the system at 12% consistency and at about F. At a point in the system reached by the stream pulp about two hours later, about 12% to 15% bleach may be added and the temperature raised to F. About of the caustic initially added will be consumed in the two-hour treatment before the bleach is added, but the stock will remain ina distinctly alkaline condition until practically all the bleach is consumed. 1

In addition to the advantages hereinbefore wood pulp fibers still have goo enumerated resulting from the application of the process of our invention, wehave found that in actual mill practice we produce a final product of higher pH value than that as- 5 sociated with pulp bleached in the usual way.

Thus, the raw. stock bleached in the usual way resulted in a dr sheet of pulp, usually termed dryer sheet in the trade, having a pH value of about 3.2 to 3.6 when tested by staining with brome-cresol green, but when processed as herein described, resulted in a dry sheet of pulp having a pH value of about 4.8 to 5.5. The higher pH value connotes greater stability in the pulp. Results 9btained from large scale tests have shown that pecial utility when applied in the treatment of raw sulphite pulp to produce a product which maybe hydrated or gelatinized as readily as the usual sulphite pulp whichhas not been subjected to the process of this invention and .is as suitable for the manufacture of hard, rattly papers of good strength and tear resistance, but of superior'whiteness and cleanliness. The initial treatment-of the raw pulp with weak solutions of alkali at about room temperature, as herein described,

does not change the-"character of the fiber as to its alpha cellulose or pentosan content to a pronounced degree, but is more especially eiiective in the removal or in facilitating the subsequent removal by bleaching, of trouble- 40 some bleach-resisting and coloring impurities which otherwise require a more drastic bleaching treatment and cause a product of inferior whiteness, cleanliness, .and copper number, to result. Our process is practica ble, in that-it may be carried out with a low alkali and bleach consumption, without the use of heat in the preliminarystep, and with-.

out requiring equipment other than that usually employed in ableaching process.

alkalinity of the dilute solution at about room temperature used in the treatment of the pulp and inmaintaining-the bleach liquor alkaline is afl'orded by caustic soda, other chemlcals, such as sodium carbonate, sodium phosphate, or borax ma be used-for this purpose." Again, while t e s cific example deals with the use of sulp ite pulp as a raw material, and aprocess comprlsinv essen- 4 tlally only the chemical treatment of lileaphin after the initial-alkali treatment, other pu psmay be used as a raw material and other chemical treatments may advantageouslyincluded. For instance, the treat- -ment ofpulp with a dilute caustic soda. solu- While in the specific example given the tion at about room temperature, followed by the addition of bleach to the solution, may

be advantageously included as the final steps in a puilpgrefiningprocess which comprises the step of digesting raw wood pulp at elevated temperature or at room tem rature in an alkaline liquor of sufiicient a alinity to refine. the pulp to highalphacellulose content, or in a? pulp refining process which comprises the step of treating the raw wood (pulp with chlorine'water or with bleach liq:

uor before digesting in the*'=-allialine refining 1i uor.

at we claim is: c I 4 1. .A process which comprises treating cellulose pulp with a dilute alkaline solution at about room temperature, maintaining the pulp in contact with the solution for a substantial period of time,'and adding bleach to the alkaline solution to whiten the pulp.

2. process which comprises treating raw cellulose pulp with a dilute alkaline solution. at about room temperature, and after a substantial amount of alkali has been consumed, adding bleach to the alkaline solution to whiten the pulp.

3. A process'which comprises treating raw cellulose pulp with dilute caustic soda solution at about room temperature, and then,

after awhile adding bleach to the solution in I amount suflicient to whiten the pulp while the solution still contains sutficient caustic soda to maintain a distinctly alkaline conditionthroughout the bleaching operation.

4."A process which comprises treating a flowable aqueous suspension of raw sulphite pulp at about room temperature with about caustic soda, based on the air-dry weight of pulp, and after a substantial amount of the caustic soda has been consumed, adding bleach to the pulp suspension.

3 5. A process which comprlses treating a fiowable aqueous suspension off-raw sulphite pulp at about room temperature withabout caustic soda, based on the air-dry weight of pulp, and after about twohours treatment, adding bleach to the pulp suspension to whiten the pulp.

' 6. In a process which involves the flow of unbleachedpulp as a continuous stream of pulp suspension through a suitable system, those steps which comprise adding caustic soda to the stream to produce a di nte solution at about room temperature, and adding bleach at point in the system, reached by the stream considerably later but while it Q still contains suificient caustic soda to maintain the pulp in a distinctly alkaline condipractically all the bleach is contion until sumed. P k v In testimony whereof we have aflixed our signatures.

GEORGE A. RICHTER.

MJLTON O. SCHUR. 

